1. Field of the Invention:
The present invention relates to the manufacture of molds made of a metal having a high melting point, more especially by spraying molten metal onto a pattern made of filled silicone elastomer.
In the description which follows, the "master model" denotes the original model, the word "pattern" denotes a replica or reproduction of the model and the "mold" or "shell" denotes the mold or a part of the mold produced from a pattern.
2. Description of the Prior Art:
The manufacture of metal molds used for the mass production of plastic articles using conventional techniques such as gravity casting, compression molding, injection molding, thermoforming, and the like, is very time consuming and costly (usually several thousands of dollars per article) as a result of the manufacture thereof by mechanical means which require many hours of machining.
The molds produced by traditional machining methods can be made of steel and can conform to highly accurate dimensions, with tolerances on the order of a few .mu.m and below. However, for many manufacturers and industrial sectors, in particular for the footwear and fancy goods industry, some sectors of the automobile industry, and the like, the dimensional tolerances are less strict and the number of different molds to be manufactured is very high.
Serious need thus exists for a process for the manufacture of molds made of a metal having a high melting point, for example, steel, which process can be used easily and quickly, without requiring costly machine tools and which enables such molds to be produced at a much lower cost than by the traditional methods.
As a result of their intrinsic physico/chemical properties, in particular because of their good thermal behavior, silicones have to date been employed in mold manufacturing processes as a constituent material of the pattern, or of the mold, with the intention of reducing the prime cost of the said molds.
Thus, in French Pat. No. 2,098,629 a silicone mold is described, permitting the molding of metals and nonferrous alloys melting at a temperature below 700.degree. C. and capable of being used at least 100 times. These molds make it possible, therefore, to only produce molded articles made of a metal having a low melting point.
In U.S. Pat. Nos. 3,638,299 and 3,784,152, which are similar in nature, a mold is produced by spraying a metal onto a pattern. The pattern may be made of very diverse materials and in column 1, line 49, of said U.S. Pat. No. 3,784,152 it is indicated in passing that the pattern may be made of silicone rubber, without further detail, and in column 1, line 65 to column 2, line 14, it is further indicated that the metal to be sprayed may be any metal. It is clearly specified, however, that the preferred metal is zinc or aluminum, the melting points of which are relatively low.
In published Japanese Patent Application, Kokai No. 54/27,394, a process is described for the manufacture of a metal mold by producing a master model made of aluminum sheet, onto which is produced a pattern made of an RTV silicone elastomer. Then, from this pattern, a metal mold is produced by spraying a metal which, according to the sole example of this application, is also zinc.
These methods of mold manufacture are of interest because they make it possible to produce, very rapidly and at a low cost, metal molds which can be employed to produce articles made of thermosetting, thermoplastic and elastomeric materials by various known molding and forming techniques.
However, since such molds are made of a metal having a low melting point, such as aluminum, zinc, tin, or their alloys, even after they have been reinforced, they do not have adequate mechanical characteristics to be repeatedly subjected to the stresses and the forces inherent in molding and forming techniques.
Good mechanical characteristics could be produced by selecting a metal or alloy other than aluminum, zinc or tin. However, other metals or alloys, such as steel, which have good mechanical characteristics, also have a high melting point, generally above 1200.degree. C., principally above 1400.degree. C. in the case of stainless steels. Now, spraying molten steel onto silicone patterns, according to the processes of the prior art, is not feasible because the molten steel droplets rebound on the pattern which, furthermore, suffers damage and is very severely distorted as a result of the very high temperature of the said droplets.